1. Field of the Invention
The invention relates to the field of wireless local area networks (WLAN), and more particularly, to a wireless system for locating mobile nodes within a monitored environment using a combination of RSSI (received signal-strength indicator) and sector discrimination using an array of limited-field-of-view antennas.
2. Description of Related Art
Wireless area networks are becoming more commonplace as wireless devices decrease in size, cost, and power consumption. As these devices slowly replace their wired counterparts in the areas of sensors and personal wireless devices, networks grow from a few devices to become facility-wide webs, where information really is only a few wireless hops away. The ZigBee wireless protocol, built on the IEEE 802.15.4 standard, allows for the easy formation of such networks and is becoming increasingly prevalent in the creation of facility-wide mesh networks. The growth of these networks provides a plethora of information that can be analyzed to determine attributes of a node (or wireless network device) on the system. One of these attributes is the location of the node.
Real-time location systems (RTLS) are used to track and identify the location of objects in real-time. A few solutions are currently present using wireless sensor networks or other large wireless mesh networks. These solutions rely on the various methods of signal-to-distance conversions such as time-of-flight arrival (ToFA), received signal-strength indicator (RSSI), and angle of arrival (AoA).
A radio location system suitable for tracking objects over shorter distances is proposed in U.S. Pat. No. 5,119,104. A fixed array of receivers is distributed over the area and a transmitter is attached to the object to be tracked. Time-of-arrival measurements of transmitted signals at the distributed receivers are used to locate the object. However, this system does not take into account multi-path effects or signal propagation differences. Additionally, ToF measurements typically require synchronous clock signals on all devices making them prohibitively complicated and expensive.
Omni-directional RSSI systems use the relative power of received signals from a plurality of nodes to determine relative distances between the nodes. This is described in U.S. Pat. No. 6,963,289 with an RFID system. However, omni-directional RSSI-based systems do not solve the problem of multipath effects or singularities that arise from these multipath effects. The typical solution to this flaw is to use so-called “finger printing” systems that require extensive site surveys to create “heat maps” of the facility. The relative signal strengths measured in real-time are compared to the heat map to determine the most likely point of positioning. Additionally, these systems typically rely on past positions as a predictor of future position. This requires that the solution be real-time in nature. Privacy concerns as well as data management concerns arise with required real-time location tracking.
AoA systems are currently too expensive due to the signal processing power needed to recognize small differences in phase. Currently, AoA systems are focusing on getting exact angles to measure location such as described in U.S. Pat. No. 7,233,799.
Another system combines multiple location estimation methods. Data from RSSI and ToFA systems are combined to form a more accurate and robust system. The patent application 2008/0130604 recognizes the problem that exists with just one of these technologies, but this idea does not address the expense of synchronized clocks that are needed in the ToFA measurements.
It is, therefore, desirable to overcome the above problems and others by providing a system and method that utilizes the potential accuracy of an RSSI-based location finding system with the reliability of a system that provides some sector or other area of location reliability.